Aspheric lenses (or more simply, aspheres) are used in a wide variety of applications as a way to optimize optical performance while ensuring an efficient design. They are characterized by a radius of curvature that varies from the center to the edge of the lens, affording them greater optical functionality. Because of this, they can reduce the number of elements needed in an optical assembly (better performance meaning the use of fewer lenses) leading to both weight and size reduction in systems.
They are particularly effective at reducing the effects of spherical aberration, and additionally help manage other higher order aberrations and can facilitate improved light focusing and collection efficiency. Their use is being stimulated by improvements in optical manufacturing processes and metrology techniques---and the demand for highly functional small optical devices.
So why does that matter? These attributes are attractive in small imaging applications where space is at a premium, and the demand is for extreme accuracy. For instance, you will find aspheres in microscope imaging objectives and other image lens assemblies in life science instruments, medical devices, semiconductor wafer inspection tools, and night vision optics. In addition, design of these complex optics and associated precision manufacturing methods have become enablers in the commercialization of advanced optical systems used in diverse applications, such as LED illumination, laser beam shaping, and lithography stepper lenses. In all these applications, the use of aspheres has added significantly to the functionality and capability of end-use products.
Refinements in optical manufacturing processes are a key driver behind the ability to produce increasingly innovative aspheres. However, making them also requires innovative metrology tools — to verify the surface accuracy of an asphere to ensure compliance with end-use requirements.
Aspheres typically exhibit complex, high-aspect ratio, sloped surfaces, and precise metrology plays a key role in providing essential quality control and feedback during the fabrication process. To measure them, it is preferable to use non-contact metrology tools such as 3D optical profilers and laser interferometers which eliminate the possibility of damage to a finished, high-quality surface, simplify alignment compared to contact tools, and who’s full-area 3D measurement provide better process insight and characterization of the surface compared to individual 2D profiles.
How Has Zygo Responded?
ZYGO has recently added aspheric “analysis” capabilities to its standard suite of 3D optical profilers and laser interferometers, reflecting the increased use of such lenses across industry as the demand for greater functionality on a reduced footprint continues to be a dominant trend. Manufacturers can now with one tool measure and analyze data from an aspheric lens, increasing the speed and cost-effectiveness of the surface verification process.
Metrology tools in particular have been developed by ZYGO with aspheres in mind, and they now for the first time come complete with analysis capabilities. The CompassTM enables a more efficient manufacturing and measurement cycle to improve yields and reduce time-to-market of ever-changing complex asphere designs – particularly the micro-aspheres used in mobile device cameras and sensors.
The small imaging device sector is highly competitive, and as such aspheric measuring and analysis tools are key enabling technologies as manufacturers jostle to produce more and more complex and innovative aspheres.